Filters of this type are frequently intended to be installed in the vena cava, such as the inferior vena cava, in particular percutaneously.
It has been known for many years that filters which are left indefinitely can, in the long-term, cause considerable complications for the patient, such as obstructions of the vessel in which the filter is implanted and side-effects in the veins.
Today, these side-effects are accepted, in particular when there is a constant risk of pulmonary embolism during the course of time.
Nevertheless, it has appeared desirable to be able to remove a "barrier" of this type, in particular, whenever the risk of the embolism recurring no longer creates a danger for the invalid.
Current temporary filters have this ability to be installed to provide protection only for the necessary amount of time and then of being removed.
Several categories of temporary filters exist already or are in the process of development, among which can be cited:
filters of which the filtering element rests on a catheter emerging from the patient's body at the location of the aperture which is provided through the skin and from which the access route to the vessel extends. In practice, these devices cannot be left in place for more than a fortnight owing to the risks of infection associated with the catheter outlet. Thus, they are usually used for medical action providing short-term protection, such as thrombolysis operations for clots in the vessels. PA1 an open structure which defines a tubular surface having a tube axis and through which the blood can pass, this tubular structure having a first, small tube diameter, for its implantation as far as said vessel, and a second, expanded tube diameter so as to be suitable for contacting this vessel radially in a position in which it is implanted therein; PA1 and means for stressing this structure, when the latter comprises its second, expanded tube diameter in order: PA1 either for at least a portion of this structure to move closer to its axis such that a constriction area corresponding to a filtering position suitable for retaining clots which may be present in the blood passing through the structure is formed; PA1 or, from this so-called filtering position, to move the portion of the structure defining this constriction area away from its axis, such that the filtering capacity of the device is varied.
Filters can also be cited of which the filter element rests on a catheter which is kept implanted under the skin, similar to the device described in patent application FR-A-2 657 261. These filters can be used for a longer duration since the risk of infection is very largely limited, or even eliminated. Nevertheless, implantation is restricted in terms of time, even though this may be a period of several weeks, in so far as, in practice, the vessel walls and the filter adhere to one another during the course of time. It may even occur that the filter can no longer be withdrawn without damaging the vessel.
A third category of filter concerns those of which the filter element is made from a material which has the capacity to be absorbed over time, such as the one proposed in patent application FR-A-2 689 388. The length of use thus depends on the speed at which the material is absorbed in the blood. Although numerous research teams are working on the subject, the Applicants are not aware of any filter of this type being currently available, however, in view of the problems of mastering the absorption process in particular.
This brief survey shows that, at present, to the Applicants' knowledge, no equipment exists which enables the blood to be filtered "in situ" for a given period of time, possibly for more than several months, without the patient having to suffer the long-term complications of the current definitive filters which can be left in place for life.